Detection of individual ions in time-of-flight mass spectrometers (TOF MS) requires nanosecond-level detector speeds and approximately 1E+6 gain. Detector's dynamic range and life time are the primary concerns. Parameters of existing time-of-flight detectors limit the combination of resolution, speed, dynamic range, and robustness. Typical TOF detectors have 1 Coulomb resource, measured by the output current, while modern pulsed ion sources may form ion fluxes up to 1E+9 ion/sec. At 1E+6 gain the detector's output current reaches 0.1 mA and, thus, lifetime is limited to 10000 second (3 hrs) only.
Dual microchannel plate (MCP) detectors are capable of sub-nanosecond-detection speeds at 1E+6 gain. However, they saturate at ion fluxes above 1E+6 ion/sec/cm2 and their resource is limited to approximately 1 Coulomb (1 C). Currently dual MCPs are used either for weak ion sources or at a reduced gain, which results in missing individual ion signals and limiting the TOF MS dynamic range.
Secondary electron multipliers (SEM) built of discrete dynodes may reach 1-2 ns time response per individual ion (the 268 model SEM by ETP is a typical example), however, ion to electron converters are of small size, are prone to non-uniform electron collection, and form bogus signals related to secondary negative ions. When SEM is exposed to a technical vacuum, its active surfaces also deteriorate at approximately 1 C output charge, and thus strongly limiting both the associated dynamic range and the associated life time.
Daly detectors employ an intermediate conversion of electrons into photons, photon detection, and electron amplification within a photo-multiplier tube (PMT). Use of sealed PMT strongly enhances the detector resource and life time. In more detail, detected ions hit a kV biased metal electrode and emit secondary electrons. Electrostatic field assists electron collection onto a scintillator. High energy electrons emit photons from the scintillator. Photons are detected by a PMT. However, such detectors were not intended for detecting fast signals. Primary converters form a large (i.e. tens to hundreds of nanoseconds) spread at ion arrival. Electron collection does spread the signal for at least several nanoseconds. Secondary negative ions form additional peaks, which are shifted in time.
Recently emerged hybrid detectors are better suited for detection of fast signals in TOF MS. They also employ electron-to-light conversion and light detection by a sealed PMT. Typical hybrid detectors comprises a single MCP (which faces the ion beam and amplifies a single ion to approximately 300-1000 electrons), a scintillator beyond the MCP, and a sealed PMT. Some fast PMT, like the 9880 model by Hamamatsu, provide up to 300 C resource at rise times in the neighborhood of 0.6 nanoseconds. However, the MCP itself is suspected to limit the maximal ionic flux and life time. Besides, electron amplification by the MCP leads to a faster degradation of the scintillator by depleting the thin metal coating on top of the scintillator and destroying the scintillator surface. Experiments have shown that commercial hybrid detectors run out much faster compared to the life time of sealed PMT.
Multi-reflecting TOF mass spectrometers (MR-TOF MS) add an additional constraint to the TOF detector—namely, there is very limited space available for the detector, and a right-angled detector is highly preferred. Recent additions to fast pulsing methods as described in WO2011135477, WO2013192161, and WO2013067366 (each of which are incorporated herein by reference) pose a requirement for detectors large ionic doses and large resource. Fast pulsing in MR-TOF MS introduces another requirement for TOF detectors—avoiding the pick-up of high voltage pulses, which frequently occur within a pulsed converter.